This invention relates to a hot melt adhesive system which, while retaining conventional hot melt heat activatable bonding characteristics also displays pressure-sensitive adhesive characteristics at ordinary room temperature.
Hot melt adhesive systems are currently being utilized to provide a substantially permanent bond between an article and a substrate. In particular, such a class of adhesives can be utilized where high strength of bonding is desirable, or even critical, to the particular application. Such applications include articles such as plastic sheeting which are to be laminated to a substrate, decorative or functional hardware applied to an item, or non-rigid photographs and art materials which are mounted on substrates so as to provide a rigid, permanent mounting therefor. Such adhesive systems, while affording a substantially permanent bond, do not allow for the temporary positioning of the item, e.g., a photograph, to insure exact placement or registration thereof on the substrate prior to permanent bonding. This is because conventional hot melt systems do not have ambient temperature tack or adhesion, i.e. pressure-sensitive characteristics.
One typical hot melt system for mounting items such as art work involves the utilization of a fibrous paper which has been impregnated with a hot melt adhesive, the paper then being inserted between the particular art work and a mounting substrate prior to bonding. Such hot melt tissues do not provide an ultimate user with an appropriate method of obtaining a positionable temporary bond prior to the achievement of a heat-activated permanent bond, which can thereby result in unacceptable misalignment of mounted articles. In most cases, it is not possible to correct such errors without damaging the art work in some fashion.
Positionable adhesives have been developed which are based upon a high tack pressure-sensitive adhesive layer. The adhesion level is reduced to provide positionability by the incorporation therein of microscopic frangible glass bubbles on both adhesive surfaces. Mounting is achieved after desired positioning of the article to the receptor surface upon the application of pressure to break the glass bubbles, thereby bringing full contact of the aggressively tacky adhesive layer to the surfaces of the article and receptor material. However, such pressure-sensitive adhesives, by their very nature produce bonds which are substantially lower in creep resistance and ultimate shear strength than those produced by typical hot melt adhesives. For this reason, such adhesive systems are unacceptable for heavy duty applications, such as large display photo or art work mountings, or general industrial fastening uses.
One bonding film displaying pressure sensitive characteristics at room temperature, but which is capable of providing a heat-activatable permanent bond, is disclosed in U.S. Pat. Nos. 3,326,741 and 3,753,755. The adhesive system described therein has sufficient room temperature tack to allow formation of a temporary bond, sufficient to hold structures to be united in a fixed position, and thereafter to be heat cured to a strong permanent bond. However, the system is limited to thermosetting resins in attaining the ultimate bond strength.
A mildly pressure-sensitive adhesive system in microsperical form which exhibits a modest degree of room temperature tack has been disclosed in U.S. Pat. No. 3,691,140. More recently, in U.S. Pat. No. 3,857,731, such an adhesive is disclosed in conjunction with a binder to produce a repeatedly usable pressure-sensitive sheet material. Such a sheet material has utility in the temporary mounting of articles where a low level of adhesion, with repetetive application capability, are the primary features. An example thereof is an adhesive-surfaced bulletin board presenting a tacky surface from which objects can be repeatedly adhered to and removed from. Such an adhesive system is not designed to provide a substantially permanent high strength bond.
It has now been found that an adhesive product can be prepared having positionable pressure-sensitive tack, which upon heat activation can undergo a transformation to form a substantially permanent high strength hot melt bond. Such a system can be prepared by the utilization of the aforementioned microspherically-shaped adhesive in conjunction with many conventional hot melt adhesive systems.